My wife says that once they're big enough that you can see their knees and ankles "articulating", the scream can not be suppressed! :)
I can't tell you now many spiders I've dispatched over the years at bedtime. My daughter always makes a quick spider-survey of the ceiling and walls before she'll go to bed.
I think those big'uns could be scary, but frankly, there's something MORE scary and gross about thin spindly legs and bodies - especially when they're a weird color! We have these weird red-legged things around our house that we find occasionally. Their bodies are grayish and all of it is transparent-like. Transparent is too creepy.
One time just as summer began, my wife and I came home and saw this monstrous spider on the wall. Both of us thought that it was a toy since it was roughly six inches in diameter. But it wasn't, it was one of those "wood spiders" one occasionally comes across. Creepy, I tell you.